1. Field of the Invention
The present invention relates to a two-component developer, a replenishing developer, and an image-forming method each of which is used in an electrophotographic system, an electrostatic recording system, or an electrostatic printing system.
2. Description of the Related Art
A print-on-demand (POD) technology has been attracting attention in recent years. The digital printing technology involves directly printing an image without through a plate making step. As a result, the technology can respond to small-lot printing and a demand within a short delivery time, and can respond also to printing in which contents vary from sheet to sheet (variable printing) and dispersion printing in which multiple output devices are activated on the basis of one piece of data by utilizing a communication facility. Accordingly, the technology has advantage over the conventional offset printing. When one attempts to apply an image-forming method based on an electrophotographic system to a POD market, tinge stability as well as the three basic elements of printing, that is, a high speed, high image quality, and a low running cost must be improved. In view of the foregoing, essential performance requested of toner is as follows: the toner achieves an image with quality and definition each of which is comparable to or higher than the conventional one without narrowing a color reproduction range, and a toner consumption is reduced. Further, the toner must respond to a reduction in fixation energy and various kinds of recording paper.
The following proposal has been made (Patent Document 1): a toner laid-on level is set to 0.35 mg/cm2 or less, and a deficiency (blister or the like) occurring at the time of fixation is suppressed while a toner consumption is reduced, whereby a high-quality, good-appearance color image stably having a wide color reproduction range is formed. According to the proposal, a high-quality, good-appearance color image which rages to a small extent, is excellent in fixing performance, and stably has a wide color reproduction range can be formed. The use of toner having a toner particle in which the amount of a colorant is increased in a conventional electrophotographic system is expected to have a certain improving effect on fixing property, but may reduce the chroma, or narrow the color gamut, of an image. A possible cause for the foregoing is as follows: as a result of an increase in amount of the colorant, the dispersibility of the colorant reduces, and the hue of the toner changes, with the result that the chroma of the image reduces, and the color gamut of the image narrows.
As described above, an increase in amount of a colorant in a toner particle is apt to reduce the density stability and gradation of an image at the time of the long-term use of toner. Conventional toner corresponds to the curve A of FIG. 1 where the axis of abscissa indicates a potential (development contrast) and the axis of ordinate indicates a density (it should be noted that the characteristic represented by the curve is referred to as “γ characteristic”). An increase in content of the colorant as compared to that of the conventional toner allows a predetermined density to be represented on recording paper with a reduced toner laid-on level, whereby gradation is represented with an additionally narrow development contrast potential (Patent Document 1). In this case, the resultant γ characteristic is represented by the curve B of FIG. 1: the γ characteristic becomes sharp, and it may be difficult to obtain high gradation. In addition, owing to the sharp γ characteristic, a change in image density due to a fluctuation in potential is large as compared to that of the conventional toner, so the stability of the image density may reduce.
The ability to obtain wide gradation and tinge stability have been essential conditions in the POD market, so development must be performed so that the γ characteristic shows a gradual slope even when a toner laid-on level is small. An increase in triboelectric charge quantity of toner with an increased colorant content is one useful approach to forming gradation by using the toner with the same development contrast potential as a conventional one. Patent Document 1 does not refer to the triboelectric charge quantity of toner, and shows no sign of actively controlling the triboelectric charge quantity.
However, an increase in triboelectric charge quantity of toner increases the electrostatic adhesive force of the toner with respect to the surface of a carrier or photosensitive member, with the result that developing performance and transferring performance reduce, and an image density reduces in some cases. There has been a proposal specifying a relationship between a toner charge quantity and an adhesive force between toner and a carrier (Patent Document 2). According to Patent Document 2, setting each of the toner charge quantity and the adhesive force within a predetermined range allows the formation of a high-quality image with no image failure. However, in the documents it is not assumed that region of a triboelectric charge quantity requested of toner with so large a colorant content that a toner consumption can be reduced, so the adhesive force between the carrier and the toner is still strong, and a sufficient image density cannot be obtained in some cases.
Accordingly, in order that an image may be formed with a smaller toner laid-on level than a conventional one, the image must be efficiently developed with toner which has a large colorant content, contains a colorant having high dispersibility, shows high coloring power, and has a high triboelectric charge quantity. Toner having the following characteristics and a developer containing the toner have been desired: the toner contains a colorant having good dispersibility, and has a high triboelectric charge quantity, a high-resolution, high-definition image can be efficiently developed with the toner, and each of the toner and the developer can stably express good image quality even when continuously used without impairing the color gamut, chroma, and lightness of the image.
Patent Document 1: JP 2005-195674 A
Patent Document 2: JP 2006-195079 A